Electrical instrument of the differential galvanometer type



Fella. 4, 1959 N. D. LAWLESS ET AL 42,75,461? ELECTRICAL INSTRUMENT OFTHE DIFFERENTIAL GALVANOMETER TYPE Filed Aug. 3, 1956 n iv: v y 1 v z WV fix/Q5 ATTORNEY United States Patent ELECTRICAL INSTRUMENT or THEDIFFER- .ENTIAL GALVANQMETER TYPE Application August 3, 1956, Serial No.601,935

1 Claim. or. 324-140 This invention relates to electrical instrumentsand more particularly to electrical instruments of the differentialgalvanometer type such as used in indicating liquid level particularlyin gasoline tanks of automotive vehicles.

The present invention is an improvement over the type of apparatusdisclosed in the United States Patent 2,139,385, granted December 6,1938, in the names of Bruce H. Schwarze and Robert H. Bigler.

In instruments of the type herein considered the magnetic flux frommagnet coils acts upon a moving element such as an armature. Between acoil of the instrument and the armature a coil end piece of iron isgenerally interposed. This end piece may be arcuate or flat butheretofore acts with the armature to define an air gap which oftenvaries in length as the armature is rotated. Because of non-uniformmagnetic field distribution due to the variable air gap, the effect ofthe flux on the armature is erratic and the operation of the instrumentis not as satisfactory as would be desired.

It has now been found that the use of a cupped end piece provides astronger and more uniform acting instrument because the air gap betweenthe end piece and the armature may be shortened from that heretoforeexperienced as necessary and that the shortened gap may be made moreuniform or substantially uniform regardless of changes in the angularposition of the armature with respect to the coil. An increase in theease of calibration and assembly also results so that a more easily readdial is permissible and stable operation of the instrument is assured.

Accordingly, an object of the present invention is to provide animproved electrical instrument facilitating ease in proper assembly andutilizing an armature and a coil for producing a flux in such a manneras to give a uniform operational effect.

A feature of the invention is an improved electrical in strumentutilizing two coils acting on an armature and in which at least one ofthe coils is provided with an end piece having a cupped symmetricalsurface facing the armature.

This and other important features of the invention will now be describedin detail in the specification and then pointed out more particularly inthe appended claim.

In the drawings:

Fig. l is a front elevation of an instrument in which the presentinvention is embodied, the dial being removed;

Fig. 2 is a bottom view of the instrument shown in Fig. 1;

Fig. 3 is a sectional viewitaken through one of the coils in Fig. 1 andis drawn to an enlarged scale; and

Fig. 4 is a perspective view of an end piece employed in the structureof Figs. 1, 2 and 3 and is drawn to the same scale as Fig. 3.

The instrument as depicted in the drawings is very similar in manyrespects to that disclosed in the United States Patent 2,139,385mentioned above. It comprises 2 a brass frame 10 to which a dial 12 isattached by means of rivets 14 and 16. A pointer 18 has an intermediateportion 18' which extends through an arcuate slot (not shown )in thedial 12 and is fixed to rotate with a fiat iron armature 20 and abalance weight 22 fixed to a shaft 24. The shaft 24 is journaled on theframe 10 in the customary manner as in bearings such as the bearing 25.Two coils 26 and 28 are fixed to the frame 10 by means of two L-shapedbrackets 30 and 32, respectively. The coils 26 and 28 are so arrangedthat their axes are di rected toward the armature 20 and its supportingshaft 24. The circuitry is such that a constant current passes throughthe wire of the coil 26 and a variable current flows through the wire ofthe coil 28 as is conventional. Terminals 31 and 33 are provided asnecessary connections through an insulating strip 35.

In the instant disclosure, the core 26 is of conventional constructionand has a cylindrical and fiat end piece 36 facing the armature 20. Thecoil 28 is also of conventional construction in that it is provided witha spool core serving to support it, but a free end of this core bears ashoulder portion 40 at one end for retaining a special end piece orconical washer 42. This washer is apertured at 43 to receive a reducedportion of the core spool and is so placed in the assembly that it isspaced from the coiled wire 44 of the coil 28 by a metal washer 46 and awire retaining paper disc 29. The cupped and symmetrical surface 48 ofthe washer 42 is so placed in the completed assembly as to face thearmature 20.

It is to be understood that upon operation. of the instru ment bypassingcurrent through the coils 2'6 and 28, the strength of the fields fromthe two coils influences the iron armature 20 and determines theposition of the pointer 18. This is the principle of operation disclosedin the Patent 2,139,385, heretofore referred to. The flux generated bythe coil 28 will be affected by the end piece 42 so that a substantiallyuniform air gap is provided between the armature 20 and the end piece asthe pointer 18 moves through its scale of readings of the instrument andregardless of the axial positioning of the end piece.

If the end piece 42 were of arcuate and elongated construction, it wouldbe necessary to maintain or adjust its rotative position on the coilaxis so that it would be in the same plane as the armature. With theinstant construction, however, no attention need be paid to thisalignrnent as the end piece will be positioned properly regardless ofinadvertent rotation on the coil axis.

The drawings show an instrument in which only one coil is provided withthis special end piece giving the uniform air gap, but it is to beappreciated that either or both coils 26 and 28 may be provided withsuch end' pieces without departing from the present invention. Thesurface 48 on the end piece is preferably made conical but it could bespherical. In any event, it should be substantially symmetrical in orderto gain the desired effect.

It has been found that in the use of the cupped end piece 42, eventhough used in combination with only one coil, the pointer movement isstronger than heretofore experienced with similar instruments of thesame general type. The pointer movement is less erratic; in other words,more stable in the pointer operation, and calibration of the instrumentis easily performed for there are no conditions under which the pointeris inclined to jump unduly.

The theory which forms the basis of the present invention may be statedin another way and that is that it concerns the maintenance of thesubstantially constant length air gap between the iron end piece in thecircuit and the moving element (the gage armature). The ideal magneticcircuit, insofar as the amount of magnetic flux is concerned, would havea closed iron path (no air gap).

Patented Feb. 24,1959

Ijoweyer, in order to have an air gap must be introduced into the ironpath. In some prior constructions, a particular scale position of thepointer and armature wcTuld give the longest air gap. A movement of thepointer in either direction from that position'would shorten the airgap. The particular position of the pointer. in such a situation would,therefore, be unstable since a magnetic circuit constantly tends toshortenits air gap. In using the present invention the air gap isapproximately the same when the pointer has movedthrough its fullarcuate range as when in any intermediate position and there is notendency for the armature and pointer to jump. We claim:

An electrical instrument including a frame, a flat armature pivoted onsaid frame for rotation in its own plane, two coils with correspondingcores supported on said the magnetic circuit do work,

frame with their axes directed toward said armature and lying in planesparallel with the plane of said armature, an end piece fixed to one endof each of said Cores and cooperating with the said afmature to definean air gap, and one of said end pieces having a cupped surface facingthe edge of said armature in defining its air gap, and said cores,armature and end pieces being of magnetic material. 7

References Cited in the file of this patent UNITED STATES PATENTS1,220,587 Beede Mar. 27, '1917 1,306,624 Porter June 10, 1919 2,040,060Middleton May 5, 1936 2,391,992 Malone Ian. 1, 1946

